Recently, there is an increasing interest in energy storage technologies with the advancement of mobile phones, camcorders, notebook computers, electric cars, etc. In particular, electrochemical devices are drawing a lot of attentions and, among them, rechargeable secondary batteries are the center of attention.
Especially, lithium secondary batteries using lithium and an electrolyte are being actively developed because they are small-sized and lightweight and are likely to realize high energy density.
As anode active materials for existing lithium secondary batteries, carbon-based compounds have been mainly used because reversible intercalation and deintercalation of lithium ions is possible while maintaining structural and electrical properties. Recently, as it is known that silicon, tin or alloys thereof can reversibly intercalate and deintercalate a large amount of lithium ions through chemical reactions, a lot of researches are underway thereabout.
Since silicon has a maximum theoretical capacity of about 4020 mAh/g (9800 mAh/cc, specific gravity=2.23), which is substantially greater than that of graphite-based materials, it is promising as an anode material. However, silicon or its alloy is problematic in that the cycle life of the battery is shortened due to degradation of the anode owing to volume change upon repeated charging and discharging. To describe in detail, during charging, as lithium ions migrate into the anode active material (silicon), the anode active material has a denser structure as the overall volume increases. During discharging, the volume of the anode active material decreases as the lithium ions are released. Since the anode active material and other components mixed therewith have different coefficients of expansion, voids are formed. Electrons cannot migrate efficiently because of the voids and, as a result, the battery becomes less efficient. In addition, cracking that may occur due to the decreased elasticity of the binder mixed with the anode active material leads to increased resistance since the cracking blocks electrical contact. As a consequence, the cycle characteristics of the secondary battery are degraded due to the degradation of the anode. This problem becomes severer as the content of the high-capacity anode active material, silicon, increases to provide a cell with a higher energy density.
A method of increasing the amount of the binder to improve adhesion of the anode active material during preparation of the anode has been attempted to improve this disadvantage. However, because the relative content of a conducting material or the anode active material is decreased, this leads to decrease in resistance characteristics, electrical conductivity, capacity and output of the battery.
Accordingly, development of a technology for improving adhesion binding between lithium metal oxide and a current collector using an adequate amount of a binder and, at the same time, improving the performance of a secondary battery such as capacity and output characteristics by increasing electrical conductivity is necessary.